The use of birefringent films as security devices has been reported in prior art. GB 2328180 discloses a security device comprising a birefringent film having regions of different thickness, such that when the material is viewed through a polariser, a pattern is exhibited. The birefringent film is e.g. a stretched polymer film like a PET film. The pattern is achieved e.g. by forming recesses in the birefringent film, or by bonding the film to a higher melting point layer as supporting layer and reducing the thickness or destroying the birefringence in selected regions of the birefringent film. Destruction of the birefringence is achieved e.g. by melting, hot stamping, thermal printing or laser writing. Reduction of the film thickness is achieved e.g. by removing parts of the film or by laser writing. The higher melting point supporting layer then holds the partially destructed birefringent film in place. GB 2328180 further reports that a reflective layer can be applied to the birefringent film after provision of the pattern, e.g. by metallisation or printing of a reflective material.
The device according to GB 2328180 has several drawbacks. For example, its manufacture is complicated and require a series of manufacturing and processing steps like forming the pattern on the birefringent film, lamination to a high melting point supporting layer and application of the reflective layer. The methods of forming the pattern are also complicated and require additional technical effort and equipment, like the use of high temperatures or lasers. Furthermore, stretched plastic films used as birefringent layers as described in GB 2328180 do generally exhibit only moderate birefringence. Therefore usually a high film thickness is required to achieve a satisfying optical effect. The film thickness is further increased by the use of the supporting layer. Thus, GB 2328180 reports devices with a combined film thickness of the birefringent and supporting layer of about 15 μm. Such a high thickness is critical especially for applications in the security sector, like e.g. security markings to be applied on documents of value or banknotes.
The aim of the present invention is to provide a birefringent marking, in particular for decorative and security applications, that does not have the drawbacks of the prior art devices, is easy to manufacture and can be applied to a broad variety of substrates, surfaces and objects.
The inventors of the present invention have found that the above aims can be fulfilled by providing a birefringent marking comprising a liquid crystal (LC) material having discrete regions with different thickness. When this birefringent marking is viewed through a polariser, an image is seen that appears to rotate when the polariser is rotated. In particular striking visible effects can be achieved by providing the birefringent marking onto a reflective layer, like for example a metal layer, and/or by providing an optical phase shift or retardation layer between the birefringent liquid crystal layer and the reflective layer. The LC material can easily be provided e.g. directly onto a reflective substrate, thus a security marking can be prepared in an easy way, without the need of complicated methods or techniques and expensive equipment.